After the discovery of an electrophotographic photoreceptor employing a charge transfer complex comprised of 2,4,6-trinitro-9-fluorenone and poly-N-vinylcarbazole, the development of organic electrophotographic photoreceptors has progressed, and continuing enhancement of sensitivity, durability, and the like of the photoreceptor has been reported year by year. Furthermore, function separating type organic photoreceptors have been developed, and hitherto, various such compounds have been reported.
Regarding charge generating substances, in order to meet requirements for consistent high speed and high sensitivity functions, azo pigments, condensed polycyclic pigments, various types of phthalocyanine pigments, and the like are reported and high sensitivity materials have been developed for each of said pigments.
However, when the enhancement of a simple charge generating efficiency is put aside, needs for the charge generating substances have not been satisfied, and the following problems have not yet been overcome. It is difficult to allow the azo pigments to exhibit a stabilized function at high levels due to difficulty in the enhancement of their purity. The phthalocyanine based pigments exhibit problems, in which the variation of their function due to ambient difference such as temperature, humidity, and the like is relatively large, and when employed repeatedly over a long period of time, a charge potential obtained immediately after inactivity is lower than that obtained in a second rotation. A first rotation, a second rotation, etc. of the photoreceptor as described herein mean that when the photoreceptor starts rotation in accordance with electrical instruction for image formation, the first rotation is designated as a first rotation, and the following rotation is designated as a second rotation, a third rotation, and the like. The aforementioned phenomenon occurs markedly during reversal development. The quality of a-copy image formed by a first-rotation of the photoreceptor is definitely inferior to those obtained by the following rotation. In addition to this, as an image problem, background stain is likely to be noticed because stain is formed on the background which is to be white.
In order to overcome these problems, necessarily, an image forming process is accepted in which charge potential is stabilized by exposing the photoreceptor without forming an image during a first and following several rotations of the photoreceptor. As a result, electrical power is wasted during idling of the photoreceptor and time is also wasted until a first image copy sheet is outputted. Accordingly, it is desired to solve the problems which are a barrier to cleaner global environment, to protect resources, and to achieve high speed processing.
On the other hand, the condensed polycyclic pigments exhibit excellent stability. However, many of them have short absorption wavelengths and have no practical sensitivity for a semiconductor laser beam which has been employed as a digital light source in recent years. Of them, bisimidazole perylene exhibits a high charge generating efficiency even in the wavelength region near 700 nm and is excellent in the stability of electrophotographic properties under variations in the environment, as well as during the use over a long period of time. However, bisimidazole perylene causes problems in which during continuous exposure with high intensity, the sensitivity decreases markedly. Accordingly usable conditions are limited because when a semiconductor laser with high intensity is employed as a light source, the sensitivity is extremely decreased.
Generally, when a plurality of pigment particles and the like are mixed and employed in an electrophotographic photoreceptor, the following are employed: each of the types of pigment particles is separately prepared and such different types of pigment particles are mixed, and a visually uniform state is obtained by employing means such as dispersion and the like, or initially, finely dispersed particles of different types of pigments are prepared by means of dispersion and the like, and mixed to obtain the uniform state. According to Japanese Patent Publication Open to Public Inspection No. 7-128889 and the like, gallium phthalocyanine and a bisimidazole perylene compound are mixed employing the aforementioned method, so that a charge generating layer is formed.
On the other hand, a method is known, in which during the preparation of particles, a plurality of materials are temporarily and uniformly mixed employing means such as dissolution and the like, and thereafter, particles composing those materials simultaneously are prepared. Examples of such means include co-deposition, acid paste treatment, and melt mixing. Examples, in which charge generating agents are prepared employing such means, are described in Japanese Patent Publication Open to Public Inspection No. 7-114196 (titanyl phthalocyanine/perylene bisimido mixing), Japanese Patent Publication Open to Public Inspection No. 9-157540 (mixing of different types of phthalocyanines), Japanese Patent Publication Open to Public Inspection No. 3-50553 (mixing of different types of phthalocyanines, and naphthalocyanines), Japanese Patent Publication Open to Public Inspection No. 7-5715 (mixing of metal free phthalocyanine/perylenecarboxylic acid diimido and diimidazole), Japanese Patent Publication Open to Public Inspection No. (titanyl phthalocyanine/metal free phthalocyanine), and the like.
However, the objects of these inventions are mainly to develop the new crystal form, to broaden the spectral sensitivity region, and to increase sensitivity, but these inventions do not disclose any means to solve the aforementioned problems of light fatigue due to high intensity exposure and variation of charging properties after repeated use.
As described above, a charge generating material employable for the electrophotographic photoreceptor has not yet been discovered which exhibits both ideal electrophotographic properties and stability.